This invention relates to a light emitting semiconductor element, and more particularly, relates to a light emitting semiconductor element such as an AlGaInp visible light emitting semiconductor laser element which is used in a light source of an optical disk system.
A conventional light emitting semiconductor laser element comprises a first conductive type substrate. A first conductive type cladding layer, an active layer, and a second conductive type cladding layer are formed on the first conductive type substrate in this order to form a double heterojunction structure. A middle layer is formed on the second conductive type cladding layer. A contact layer is ford on the middle layer. The middle layer eases an energy barrier of a valence band at boundary between the second conductive type cladding layer and the contact layer. Such a laser element is disclosed in Japanese Examined Patent Publication No. Tokyo Hei 8-15228 (or Japanese Unexamined Patent Publication JP-A No. Tokkai Sho 62-200786).
The light emitting semiconductor laser element is attached to a heat sink or a stem at high temperature to form a laser device. Since the light emitting semiconductor laser element has a heat expansion coefficient which is different from those of the heat sink and the stem, the light emitting semiconductor laser element receives stress from the heat sink or the stem at ambient temperature. The stress causes deformation in the light emitting semiconductor laser element. The deformation increases a driving current, which is used for driving the light emitting semiconductor laser element. Each light emitting semiconductor is different from other light emitting semiconductors in size of the deformation. Accordingly, the driving current is not common to all light emitting semiconductor laser elements which have the same structure. In other words, electrical characteristics of the light emitting laser elements are unequal.